1. Field of the Invention
The present invention relates generally to imaging devices for nuclear medicine, and more specifically relates to gamma-ray or scintillation cameras and methods of obtaining images from radiation data acquired by such cameras by compensating for scattered radiation detected by the camera.
2. Background and Prior Art
A gamma-ray or scintillation camera as utilized in nuclear medicine is a well known device. The original scintillation camera or "Anger camera" (named after the inventor) is described in U.S. Pat. No. 3,011,057. The Anger camera uses a scintillation crystal, such as a NaI crystal, which absorbs incident gamma rays from the object under study and interacts with the gamma ray to produce light events. An array of photomultiplier tubes is placed adjacent to the crystal in order to detect and amplify these light events so as calculate the spatial location and energy level of the incident gamma ray to produce a two dimensional image of the object which then may be displayed on a CRT or printed as a hard copy.
When a nuclear medical image is being acquired, a radioisotope has been introduced into the body as a radiopharmaceutical having an affinity for certain parts or organs of the body, and the diagnostician is interested in the distribution of that radiopharmaceutical within the body or organ under evaluation. It is therefore desirable that the image accurately represent the spatial distribution of the radiation emitted from the body. When radioactive nuclei decay, a gamma-ray or high energy X-ray is emitted from the location of the decay. The gamma-rays (or X-rays) travel in a straight line until they are either scattered or absorbed. If a gamma-ray is absorbed in the scintillation crystal of the camera and detected as a light event without having undergone an intervening scattering event, then the location at which the gamma-ray was detected represents the actual location of the decay, and hence part of the distribution of the radioisotope. Such an event is considered a "good." detected event and is used to form an accurate picture of the radioisotope distribution within the body. However, if the gamma-ray scatters within the body at some point between its emission from the location of decay and its detection in the scintillation crystal, then the location at which the scattered gamma-ray is detected does not represent the location from which the gamma-ray was emitted, and thus the inclusion of such an event in the image will falsely indicate the presence of radioisotope where, in actuality, there may not have been any radioisotope. Such an event is known as a "bad" event.
The phenomenon by which a gamma-ray collides with an electron (of an atom of the body, for example), loses some of its energy and changes its direction of travel is known as Compton scattering. Because the scattered gamma-ray energy is lower than the energy of the unscattered gamma-ray, it is the energy of a gamma-ray event that is used to discriminate among detected gamma-ray events so as to include only unscattered gamma-ray events in the image being acquired. When a single gamma-ray is absorbed in the scintillation crystal, a fraction of the deposited energy is emitted as scintillation photons which have wavelengths within the visible spectrum. Because the scintillation photons are emitted isotropically from the point of absorption, only a small amount of the emitted photons reach the photomultiplier tubes (PMTs). The fraction of the total amount of photons reaching the photomultiplier tubes that produces an electrical signal in any one photomultiplier tube is dependent on the position of that photomultiplier tube relative to the location of the light event, local variations in physical properties of the crystal, reflective surfaces, other transparent media such as lightpipes, and the interfaces between all of these materials and the boundaries of the detector. Additionally, the probability that a scintillation photon entering a photomultiplier tube will be converted into an electrical pulse is dependent on local variations in the photocathode of the photomultiplier tube. This probability is known as the quantum efficiency of the photocathode. The quantum efficiency is highly dependent on the thickness and composition of the photocathode, and is thus variable from PMT to PMT as well as locally within a PMT.
The sum of all electrical pulses produced by the PMTs of a scintillation camera is proportional to the fraction of photons reaching the PMTs, which is proportional to the total number of photons emitted from the crystal as a result of interaction with an absorbed gamma-ray, and which is thus also proportional to the energy of the absorbed gamma-ray. This proportionality, however, includes a statistical uncertainty as a result of the random nature of the scintillation photon production, collection and conversion to electrons. Thus, the sum signal of all electrical pulses from the PMTs in response to a light event in the crystal, rather than representing the actual energy of the absorbed gamma-ray, merely represents a sample from a statistical distribution function which describes the relationship of the sum signal to the energy of the gamma-ray. For example, a million gamma-rays having identical energies, being absorbed in exactly the same position in the crystal, and with a constant arrangement and properties of the PMTs, will produce a distribution of measured sum signals that is approximately Gaussian (if the mean number of detected photons is greater than 20 as it would be for all practical detected events). At a different position within the detector crystal, however, a different Gaussian distribution would be measured because a different mean number of photons would be produced as a result of local variations in the properties of the crystal and variations in the materials and thicknesses of the photocathodes of the PMTs. The integral of the sum signal is known as the energy signal, which is proportional to the total number of scintillation photons emitted by the gamma-ray and thus is proportional to the energy of the detected gamma-ray. Measurement of the energy signal is used to determine the probability that the detected gamma-ray is scattered, and therefore should not be part of the acquired image, or the probability that the gamma-ray is not scattered and should contribute to the image being formed.
It is conventional in present commercial gamma-ray cameras to measure the energy of the detected gamma-rays, correct the measured energy to account for spatial variations in signal generation, and then to include in the image only those events whose energies fall within a window, or narrow range, of energies. Various methods of compensating for the spatial energy variation have been proposed and utilized in commercial cameras. George, Raynaud, and Soussaline, Correction Automatique de la Dependance en Position de 1'Energie, 11.sup.e Colloque International Sur la Fixation Renale du Hg Radioactif, Paris, 24-25 Oct. 1975, used spatially dependent, multiplicative correction factors which were stored in a look-up table according to the spatial position of the detected event and which were multiplied by the energy signal so that the mean of the energy distribution would be the same for any position within the detector area. U.S. Pat. No. 4,095,108 disclosed the use of an offset, or additive, correction factor that could be applied to either the energy signal or the energy window. However, application of a fixed width energy window to a spatially corrected energy signal, while taking into account the spatial variations in the energy distribution and thus minimizing the width of the energy window, fails to eliminate many spurious gamma-ray events which have scattered through small angles, because such events still have energies which fall within the energy window. In fact, use of a minimum width energy window actually excludes some unscattered events from the image because such a window by nature must cut off the tails of the photopeak distribution function of the unscattered event energies.
Methods have been proposed to make more accurate corrections to eliminate the contribution of Compton scatter events to an image. For example, U.S. Pat. No. 4,839,808 to Koral et al., incorporated by reference herein, discloses a method wherein the energy spectrum of events is analyzed to separate Compton scattering components from the unscattered components. This method uses a fitted shape for the unscattered components of the spectrum; however, the correction is performed after a spatially dependent energy correction has already been applied. There remains a need for improvement in the art so as to eliminate more Compton scatter events from an image, while including more unscattered events for contribution to the image.